Ordered intermetallic alloys based on trinickel aluminide (Ni3Al) have unique properties that make them attractive for structural applications at elevated temperatures. They exhibit the unusual mechanical behavior of increasing yield stress with increasing temperature whereas in conventional alloys yield stress decreases with temperature. Trinickel aluminide is the most important strengthening constituent of commercial nickel-base superalloys and is responsible for their high temperature strength and creep resistance. The major limitation of the use of such nickel aluminides as engineering materials has been their tendency to exhibit brittle fracture and low ductility.
Recently alloys of this type have been improved by the additions of iron to increase yield strength, boron to increase ductility, and titanium, manganese and niobium for improving cold fabricability (U.S. patent application Ser. No. 519,941 filed Aug. 3, 1983, Ductile Aluminide Alloys for High Temperature Applications, Liu and Koch). Another improvement has been made to the base Ni3Al alloy by adding iron and boron for the aforementioned purposes and, in addition, hafnium and zirconium for increased strength at higher temperatures (U.S. patent application Ser. No. 564,108 filed Dec. 21, 1983, Ductile Aluminide Alloys for High Temperature Applications, Liu and Steigler). Further improvements were made to these alloys by increasing the iron content and also adding a small amount of a rare earth element, such as cerium, to improve fabricability at higher temperatures in the area of 1,200C, (U.S. patent application Ser. No. 730,602 filed May 6, 1985, High-Temperature Fabricable Nickel-Iron Aluminides, Liu). Most recently, these alloys were improved by the addition of chromium to increase ductility at elevated temperature in oxidizing environments (commonly assigned and co-pending U.S. patent application Ser. No. 786,562 filed Oct. 11, 1985, Nickel Aluminides and Nickel-Iron Aluminides for Using in Oxidizing Environments, Liu). These U.S. patent applications are incorporated herein by reference. These alloys have many favorable characteristics including improved ductility, tensile strength and resistance to oxidation, all of which are desirable for structural use in advanced coal conversion systems. However, another property necessary to such uses is a resistance to sulfur attack.